double-continent is a hotly debated research area that seems to pivot on new findings every few years. Most researchers agree that a critical part of the story began during the end of the Ice Age, when a vast land bridge called Beringia connected modern-day Siberia to modern-day Alaska. People crossed the bridge and moved into the Americas somewhere between 15,000 and 18,000 years ago.

Later, as temperatures warmed, melting the glaciers that blocked coastal routes, these founders spread out. The first inhabitants of Central and South America probably descended from the earliest groups that crossed the bridge and followed the Pacific coast southward as the glaciers melted. But other specifics — like when migrations occurred, how many different groups migrated, and how populations spread out, especially across North America — are contentious.

Researchers scour anthropometric, archaeological, linguistic, and even climatic data for hints to these migrations. But for some of the most controversial questions — including whether one or many migrations occurred— scientists have been turning to genetics. Researchers have been excavating mitochondrial DNA of modern Native Americans to look for answers. Sequences of mitochondrial DNA can be linked to a person’s ancestral heritage, and differences among genetic sequences can show when variations occurred and divided populations. A recent study of nuclear gene variation in Native American populations, for example, presented data that supported a a scenario known as the “tripartite migration model,” which hypothesizes three separate Asian gene pools as sources for Native Americans.

But a model that supposes three streams from Asia to the Americas is simplistic, report geneticists Antonio Torroni from the University of Pavia and Alessandro Achilli from the University of Perugia in Italy in a study published August 12 in the Proceedings of the National Academy of Sciences. They studied a total of 84 mitochondrial genomes (or “mitogenomes”) and came up with evidence for a much more complex migration pattern for the Native Americans founders.

Their study confirmed the well-established first wave from Beringia that traveled south along the Pacific coast and reached South America. But in addition to that first wave, “we believe that more or less at the same time there was an inland migratory event, also from Beringia, which entered North America not along the coast but through the ice-free corridor in western Canada,” says Torroni. The genes of these settlers remained confined in the northern part of North America. The researchers also found evidence for a much later migration that doubled back over Beringia and further population movements restricted to the extreme north.

“Our analyses of mitogenomes from North America reveal at least four migratory events,” Achilli says.

He also says that mitochondrial DNA offers researchers the possibility of going deeper within these major groups that share genetic sequences, or haplogroups. “Each of the major Native American mitochondrial DNA haplogroups can be dissected into clades of younger ages and more limited geographic distributions.” Torroni adds: “The identification of these clades, surveys of their geographical distributions, and their dating will help to define ancient — and also more recent — migratory events.”